Preparation of silica gels

ABSTRACT

SILICA XEROGELS HAVING A NARROW PORE DIAMETER DISTRIBUTION IN THE RANGE OF 300-600A., SULFACE AREAS IN THE RANGE FROM 200 TO 500 M.2/G., AND PORE VOLUMES IN THE RANGE OF 2.0-3.5 CM.3/G. AND A PROCESS FOR PREPARING SUCH SILICA XEROGELS COMPRISING CRITICALLY CONTROLLED STEPS OF PRECIPITATION OF THE SILICA GEL, AGEING THE PRECIPITATED SILICA HYDROGEL SLURRY, WASHING THE AGED PRODUCT, AND REMOVING THE WATER FROM THE GEL WITH, WHERE NECESSARY, DURING THE GEL. THE WATER IS REMOVED IN SUCH A MANNER THAT THE SURFACE TENSION EXERTED UPON THE GEL, DURING DRYING, IS LESS THAN THAT SURFACE TENSION WHICH WOULD RESULT FROM THE DIRECT CONVERSION OF LIQUID WATER TO WATER VAPOR. TYPICAL TECHNIQUES WHICH MAY BE EMPLOYED ARE: (A) DISPLACING THE WATER IN GEL WITH AN ORGANIC LIQUID SUBSTANTIALLY COMPLETELY MISCIBLE WITH WATER AND THEN DRYING THE GEL; (B) VACUUM FREEZE-DRYING THE GEL; OR (C) ADDING A WATERIMMISCIBLE SOLVENT, WHICH FORMS AN AZEOTROPE WITH WATER WHEN DISTILLED, TO THE GEL, DISTILLING THE AZEOTROPE TO REMOVE SUBSTANTIALLY ALL OF THE WATER FROM HE GEL UNDER A SPECIFIC CONDITIONS AND DRYING THE GEL FROM ANY REMAINING SOLVENT.

June 25, 1974 Filed May 2'7, 1972 H. A. ABOUTBOUL ETAL PREPARATION OF SILICA GELS CUMULATIVE OF SURFACE AREA 5 8 8 S g 8 5' 3 3 4 Sheets-Sheet 1 SILICA GEL PORE SIZE msmsunow June 25, 1974 H. A. ABOUTBOUL ETAL 3,819,811

PREPARATION OF SILICA GELS Filed May 27, 1972 4 Sheets-Sheet 2 EFFECT OF SiO CONCENTRATION ON THE PROPERTIES OF SILICA GELS FIG. 2 A

/F' I DJ E g 22 CONSTANT PREcIPITAT ION CONDITIONS 5 2.0 II TEMPERATURE OF sc g 2) AcIU ADDITION RATE OF I.o HOURS 3 T0 40% STOIC I I 6 8 Ib II 8102 CONCENTRATION I%I 30 EFFECT OF ACID AUUITIUM RATE 0M PROPERTIES OF sIucA GEL 700 FIG. 3

Lu I a: 5 cUMsTAMT PRECIPITATION CONDITIONS I 20 II 85% SiOz 5003 2I 5C TEMPERATURE i l l l I 0 Us l.O Is 2.0

ACID ADDITION RATE (HOURS T0 40% mm EFFECT OF PREcIPITATIoII TEMPERATURE 0N TIE PRoPERTIEs OF SILICA GEL 3.0 FIG. 4 I I m0 w 500 E g W 5 C; I0 I 400 g coMs AMT PREcIPITATIoM coMoITIoIIIs L I) 8.5 I. Si O I I g 2) Ac") ADDITION RATE OF I.o HOURS I 3 P I I U; 40 so 2'0 3'0 TEMPERATURE IcI June 1 H. A. ABOUTBOUL ET AL 3,819,811

PREPARATION OF SILICA GELS Filed May 27 1972 4 Sheets-Sheet 3 3o 40 50 PORE VOLUME SHRINKAGE FIG. 5

8 a a 2 s a 2 :2 a I auomv m 21mm 1M June 25, 1974 H. A. ABOUTBOUL ETA!- 3,819,811

PREPARATION OF SILICA GELS 4 Sheets-Sheet 4 Filed May 27, 1972 5525a wmom NQE M25 Ewiom 3,819,811 PREPARATION OF SILICA GELS Henri A. Aboutboul, Brussels, Belgium, Jerome H.

Krekcler, Cincinnati, Ohio, and William Kirch, Clinton, Iowa, assignors to National Petro Chemicals Corporation, New York, N.Y.

Continuation-impart of applications Ser. No. 750,733, now Patent No. 3,652,214, and Ser. No. 750,734, now Patent No. 3,652,215, both filed Aug. 6, 1968, Ser. No. 766,693, Oct. 11, 1968, now Patent No. 3,652,216, and Ser. No. 70,621, Aug. 14, 1970, now abandoned. This application Mar. 27, 1972, Ser. No. 238,313 The portion of the term of the patent subsequent to Mar. 28, 1989, has been disclaimed Int. Cl. C01b 33/16 US. Cl. 423338 17 Claims ABSTRACT OF THE DISCLOSURE Silica xerogels having a narrow pore diameter distribution in the range of 300-600 A., surface areas in the range from 200 to 500 m. /g., and pore volumes in the range of 2.0-3.5 cm. g. and a process for preparing such silica xerogels comprising critically controlled steps of precipitation of the silica gel, ageing the precipitated silica hydrogel slurry, washing the aged product, and removing the water from the gel with, Where necessary, drying the gel. The water is removed in such a manner that the surface tension exerted upon the gel, during drying, is less than that surface tension which would result from the direct conversion of liquid water to water vapor. Typical techniques which may be employed are: (a) displacing the water in the gel with an organic liquid substantially completely miscible with water and then drying the gel; (b) vacuum freeze-drying the gel; or (c) adding a waterimmiscible solvent, which forms an azeotrope with water when distilled, to the gel, distilling the azeotrope to re- 'move substantially all of the water from the gel under a specific conditions and drying the gel from any remaining solvent.

CROSS-REFERENCE TO RELATED APPLICATIONS This application is a continuation-in-part of the following copending applications:

Serial No. U.S. Pat. No.

Now abandoned.

all entitled Preparation of Silica Gels."

BACKGROUND OF THE INVENTION 1. Field of the Invention 2. Description of the Prior Art Much of the technology in the area of silica gels involves the problem of making strong hard gel masses which will not shrink and crack upon being dried. The gels are useful as absorbents and catalyst bases. Gels of small particle size and narrow particle size distribution,

United States Patent 01 lice 3,819,81 l Patented June 25, 1974 about 50 microns mean diameter, have been found to be the most desirable for fluid bed or stirred reactor catalyst operations because they are more easily fluidized, reduce the amount of erosion of equipment and suffer less degradation by abrasion and particle rupture.

The only silica gels thus far reported having suitable particle size and narrow particle size distribution for the purposes mentioned above are characterized by a maximum pore volume of about 1.8 to 1.9 cm. g. and surface area of from about 250 to 350 mP/g. Silica gels having higher average pore diameters reported in the prior art have only been obtained as aerogels. Aerogels are subject to shrinking when they are wet due to coalescence of the ultimate particles. This produces a loss of porosity and makes the products unsatisfactory for certain uses, especially as polymerization catalysts.

The drying of a silica gel in order to avoid shrinkage has been described by Kistler in US. Pats. 2,093,454 and 2,249,767. These patents describe a process of heating the gel under sufiicient pressure to provide evaporation within the gel, without gas phase formation until the critical temperature of the liquid is reached. At this point, the liquid phase is converted to the gas phase instantaneously, thus avoiding a gas-liquid interference. Once the gas phase has been reached, the gas may be withdrawn without causing any collapse of the solid structure. However, when the liquid phase of the silica gel is water, this process cannot be used since such gels ordinarily undergo dissolution before the critical temperature is reached.

SUMMARY OF THE INVENTION The present invention provides silica Xerogels having a narrow pore diameter distribution primarily in the range of from 300 to 600 A., surface areas ranging from 200 to 500 rn. /g., pore volumes of from about 2.0 to about 3.5 cm. /g., and stability at temperatures of up to about 2,000 F. in a fluidized bed. The invention further contemplates a process for preparing the foregoing silica xerogels which comprises the following steps:

1. Precipitation of a silica gel by reaction of a water soluble silicate to precipitate the silica therefrom, e.g., by neutralization of an aqueous alkaline silicate solution, most preferably, xNa O.ySiO with a strong acid, a weak acid such as CO ion exchange resins or by any other suitable neutralization procedure, carried out in about stoichiometric proportions under good agitation, e.g., high shear agitation. The primary criterion which any silica source must exhibit to be useful in the present process is that it must permit the silica to be introduced into the aqueous system in the form of a Water soluble silicate which can yield very small dense particles of silica which gel or precipitate to yield the hydrogel. For instance, a water soluble alkaline silicate such as a sodium silicate can be neutralized by a controlled acid addition, e.g., with sulfuric acid to yield very small dense particles of silica which in turn gel to give the hydrogel. The acid neutralization will also yield an acid-base salt, for instance, sodium sulfate from the neutralization of a sodium silicate with sulfuric acid. A part of such a salt becomes entrapped in the hydrogel during precipitation and must be washed therefrom as described in step 3.

For commercial use, water soluble silicates are commonly available as alkaline silicates, including the sodium silicates, the potassium silicates, the crystalline quaternary ammonium silicates, the lithium silicates, etc. The sodium silicates are preferred as they are inexpensive and readily available at varying silicate contents, depending upon the producer. The following discussion will be in terms of the precipitation of the hydrogel from an aqueous alkaline silicate solution. The water soluble silicate selected must be such that precipitation is in accordance with the following conditions to provide a silica xerogel of the necessary parameters which is especially suited for use as a catalyst:

(a) The SiO concentration in the final slurry is between 5 and 12 percent by weight and preferably from about 8 to about 9 percent. Above an SiO concentration of 12%, the pore volume of the gel is too low while below 5% results in a gel whose structure is very fragile and cannot be practically preserved.

(b) The neutralization medium is added at a rate such that the gel point is reached in from about 30 minutes to about 120 minutes, and preferably about 60 minutes, whereafter the remaining amount of the neutralization medium required to provide a pH as called for in step 1(d) is added. Usually up to about 40 percent of the required amount of the neutralization medium is added in from about 30 to about 12 minutes, whereafter the remaining about 60 percent or more of the neutralization medium is added. Since the neutralization is exothermic, the post-gel point addition of the balance of the neutralization medium should not be at a rate which causes the temperature to rise outside of the range specified in step 1(c). The post-gel point rate of neutralization medium addition should also not be so rapid as to cause large localized areas where the pH drops below the lower limit of the range specified in step 1(d). Typically, at least 20 minutes serves as a practical minimum time for post-gel point neutralization medium addition. Long addition periods can be used but are unnecessary so long as the above temperature/pH criteria are met. Preferably, the post-gel point addition of the neutralization medium is accomplished in from about 20 to 90 minutes, and preferably about 45 minutes.

(c) The temperature during precipitation is maintained between about 0 to about 17 C., preferably 0 to about 10 C. and most preferably at about C.; and

(d) The pH of the end product of the neutralization (pre-gel and post-gel acid addition) is from 3 to 8 and preferably between about 5 to about 7.

2. Ageing the precipitated silica hydrogel slurry under the following conditions:

(a) At a pH of from 3 to 8, preferably between about 4 and about 7, and preferably at a temperature between about 50 C. and about 100 C., most preferably between about 90 C. and about 95 C.

(b) For a period of time sufiicient to strengthen the fragile gel structure, preferably from 1 to about 6 hours and more preferably from about 2 to about 4 hours.

The ageing of step 2 is necessary to strengthen the fragile gel, which will otherwise be destroyed during subsequent processing. It is critical during ageing that:

(l) the pH range be maintained in the range of 3 to 8;

and

(2) the pH, temperature and time be correlated in a very specific manner.

Amplifying upon the need for strict pH control, as the pH of the hydrogel slurry approaches 3 the qualities of the final gel begin to deteriorate and it is impossible to obtain a final gel of the desired properties. Thus, ageing at a pH lower than the pH 34 range is prohibited. On the other hand, as the pH becomes too alkaline, the properties of the final gel also suffer. The upper pH limits can only be defined with an operating temperature. At the higher acceptable temperature levels, i.e., the 90 C.+ range (considered most desirable since low temperatures significantly increase the required time), as the pH approaches 8 the properties of the final gel also suffer, and ageing at a pH greater than the pH 7-8 range is prohibited. If the temperature of heat ageing is decreased, the allowable pH of ageing may be increased over the values stated.

With respect to the correlation of the pH, temperature and time, the following relationships have been established.

If the pH is lowered at constant temperature, the time of ageing must be increased. If the pH is lowered at constant ageing time, the temperature must be increased. Obviously, both time and temperature can be increased to obtain the same effect. The reverse is true for ageing at higher pHs.

If pH is constant, lowering the temperature of ageing requires longer ageing times to be used and raising the temperature of ageing requires shorter ageing time to be used.

Considering the above factors, pH is usually the parameter held constant during the ageing as variation from the described range destroys the gel. Operation will generally be in the pH 5 to 7 range to allow a safety factor between the pH limits of 3 and 8.

With the pH in the 5 to 7 range, the temperature is almost always maintained above 50 C. and thus the preferred ageings may be considered heat accelerated ageings. While ageing will proceed at much lower temperatures, at a pH of just above 3 and a temperature in the area of 17 C. ageing will take weeks, which is commercially unacceptable. While the maximum ageing time and the minimum ageing temperature are thus not overly critical per se, they must be appropriately correlated with pH.

3. Reducing the alkaline material concentration of the hydrogel by washing the hydrogel with a liquid which is capable of displacing the acid-base salt entrapped in the hydrogel during precipitation until the resulting wash liquor at equilibrium contains less than about 20 p.p.m. (weight parts) of the alkaline material expressed as the acid-base salt. For the neutralization of a sodium silicate with sulfuric acid, less than 20 p.p.m. sodium expressed as sodium sulfate, when the sodium in the product is in equilibrium with the wash liquor at room temperature, i.e., less than about 1,500 p.p.m. (weight parts) sodium expressed as sodium sulfate in the dried gel. The product is then preferably washed with Water if water is not the displacing liquid. For certain end uses, e.g., where the dry gel is to be calcined at high temperature, say 1,800 F., for activation as a catalyst base, it is preferred to wash to less than 1,000, more preferably 600 p.p.m. (weight parts) of the alkaline material in the dried gel. Generally, the higher the calcination temperature the lower the amount of alkaline materia'l permitted in the final gel. Preferred washing liquids are solutions of salts such as aluminum chloride or sulfate and ammonium chloride or nitrate. The displacing can also be accomplished by washing with an acid such as hydrochloric,'sulfuric, or nitric acid or with water, though washing only with water is more time consuming. Dilute solutions of these acids must be used so the lower pH limit of 3 referred to previously not be exceeded, i.e., the pH does not fall below 3.

In those instances where the water soluble alkaline silicate has been neutralized with an ion exchange resin, it will, of course, be necessary to separate the hydrogel from the then alkaline form of the ion exchange resin.

4. Removing the water from the gel where the water is removed in a manner such that the suface tension exerted upon the gel by the water, during removal, does not cause excessive shrinkage of the .gel. The water cannot be removed by directly converting the liquid water into pure water vapor, such as by direct evaporation of the water by heating the gel, since one cannot then obtain a gel of the desired properties. Any method wherein the surface tension of the water in the gel is lowered sufficiently, such that no excessive shrinkage occurs when the water is re moved, is operable according to the present invention.

There are several preferred techniques for removing water from the gel; specifically, (a) a solvent displacement technique, (b) a freeze drying technique, and (c) an azeotropic distillation technique. More specifically, these methods comprise the following steps:

(a) Solvent displacement- (l) Displacing the water from the gel by the use of an organic water-miscible liquid, e.g., a surfactant, an alcohol, a ketone or other water-miscible organic compound which displaces water and has a lower surface tension than water, i.e., which will reduce the surface tension in the pores of the gel. The preferred alcohols are the lower aliphatic alcohols which contain less than about 6 carbon atoms, with the C to C monohydric alcohols being most preferred. The preferred ketones are acetone and methyl ethyl ketone, with the former being most preferred.

(2) Drying the gel by conventional procedures, e.g., drying under vacuum at about 400 F. The amount of water-miscible displacing organic liquid remaining after drying is not overly critical, as no matter when the liquid is removed it will exhibit a low surface tension on the silica gel. Generally speaking, however, most users will desire a dry silica gel product and little is .gained by not removing substantially all of the water displacing organic. liquid.

(b) Freeze dryingVacuum freeze drying the product. (c) Azeotropic distillation (1) Adding a water-immiscible solvent which forms an azeotrope with water when distilled and distilling the azeotrope so as to remove substantially all the water under specific conditions.

(2) Drying the gel from the solvent by distilling the excess solvent.

Any one of the above-described preferred water'removal procedures produces a silica xerogel having the above-stated characteristics.

The above procedures provide a silica gel in accordance with the described parameters. However, many users will desire a silica gel of a particular particle size, e.g., one adapted to serve as a catalyst support for use in fluidized beds. Accordingly, one may wish to comminute or homogenize and reduce gel particle size by high shear mixing. This optional step may be done by high shear mixing the product from step 3 to obtain particles having a desired mean diameter, for instance, a mean diameterof less than about 200 microns, preferably in the range of from 30 to 150 microns. If, however, good agitation is used during precipitation, e.g., high shear agitation, and careful pH, temperature and time control is maintained on the intermediate product during ageing, usually but not necessarily combined with agitation during ageing, a separate comminuting step can be omitted as the particles will have a mean diameter of less than about 200 microns.

5. A comminution or homogenization may be necessary in some instances where agitation is not carried out during the ageing of step 2 and process conditions are not critically observed. In such a case, the required comminution may be carried out either prior to alkaline moiety displacement, intermediate alkaline moiety displacement and water removal or after both alkaline moiety displacement and Water removal. It is preferred to comminute at a point no later than alkaline moiety displacement as small particles are much easier to wash.

BRIEF DESCRIPTION OF THE DRAWINGS FIGS. 1, 6 and 7 show the pore size distribution for the silica xerogel produced by the process of the present invention;

FIG. 2 shows the effect of the final SiO concentration in the slurry upon the surface area and pore volume of the product silica xerogel, immediately after precipitation, produced by the process of Example X;

FIG. 3 shows the effect of the rate at which the neutralizing medium is added to the aqueous sodium silicate solution upon the surface area and pore volume of the product silica xerogel, immediately after precipitation, produced by the process of Example XI;

FIG. 4 shows the effect of varying the temperature of precipitation upon the porosity (i.e., pore volume and surface area) of the gel immediately after precipitation (see Example XII);

FIG. 5 shows the variation in pore volume shrinkage upon drying versus the water content of the organic solvent from which the gel is dried (see Example II); and

FIG. 8 is a schematic illustration of the azeotropic distillation drying technique of the present invention (see Example V).

DETAILED DESCRIPTION OF THE INVENTION The invention will now be described in greater detail with reference to specific examples which illustrate the criticality of process conditions in order to obtain a silica xerogel product having a narrow pore diameter distribution primarily in the range of from about 300 to about 600 A., a surface area in the range of from 200 to 500 mi /g., pore volumes of from about 2.0 to about 3.5 cm. g., and stability at temperatures up to 2,000 F. in a fluidized bed.

Preferred silica xerogels in accordance with the present invention have a pore diameter distribution wherein above about 70% of the cumulative pore volume is within the pore diameter range of 300 to 600 A. and a total pore volume of 2.2 to 3.0 cmfi/g.

The properties of the silica xerogels obtained in accordance with the invention and particularly the porosity characteristics are discussed in terms of pore volume (PV), surface area (SA), average pore diameter (Av. PD), where and pore size distribution. FIGS. 1, 6 and 7 show the pore size (i.e., pore diameter) distribution of the silica xerogel of the present invention, in relation to, respectively, cumulative percent of surface area, cumulative percent pore volume and cumulative pore volume. Determinations of the values for the various properties are made by a nitrogen absorption-desorption technique well known in the art and described in detail in the literature. For example, descriptions of the nitrogen absorption-desorption procedure for evaluating porosity of silica gels are found in the Journal of the American Chemical Society, Volume 60, page 309 (1938), "Journal of Catalysis, Volume 2,

page 111 (1955 and elsewhere.

In the following examples, Examples I, III and V are descriptions of preferred methods for carrying out the process of the invention to obtain a silica xerogel of the stated characteristics, employing, respectively, as the water removal technique, the solvent displacement method, the freeze drying method and the azeotropic distillation method. The remainder ofthe examples illustrate the criticality of various process conditions or describe suitable alternative procedures for obtaining the silica xerogel.

EXAMPLE I 10,080 g. of sodium silicate solution containing 28.7% SiO and 8.9% of Na O was added to 12,720 g. of Water and cooled to 5 C. under agitation.

11,200 g. of H2804 (12.75 wt. percent) was then added as follows:

(a) 4,480 g. Was added at a constant rate over a period of one hour to reach the gel point; apd (b) the remainder was added over a period of 45 minutes.

The final pH of the slurry was 6.2 and the SiO; content was approximately 8.5%.

The slurry at a pH of 6.2 was then heated to C. and held at that pH/temperature for 3 hours. The slurry was washed with a solution of 1,113 g. of NH NO in 45 gallons of water, and then with de-ionized water until the wash liquor titrated or contained less than 20 ppm. Na SO at equilibrium. To illustrate all features of the process, the product was then homogenized. The slurry can also be homogenized prior to washing with N'I-I NO or can be homogenized after being reslurried in acetone.

Homogenization prior to washing (if homogenization is necessary) is most preferred as small particles are easier to wash to the required sodium level. Homogenization after acetone treatment is non-preferred due to the extra care required if any flammable vapor, e.g., acetone vapor, is produced.

The product was then reslurried inacetone, a water miscible organic solvent, and washed with acetone until the water in the acetone titrated less than 1.0% at equilibrium. The acetone was then distilled off to reduce the acetone content to less than about 1% by weight resulting in an essentially dry product.

In a preferred embodiment, for certain end uses, the following optional step can be performed in the process of this invention:

6. Calcining the product from step at a temperature between about 500 F. and about 1,200 F., preferably at about 1,000 F. This final calcining/drying removes all traces of the water displacing organic liquid, e.g., acetone. It should be noted that this final calcination/ drying at 500-1200 F. is generally not considered a catalyst activation (which normally requires higher temperatures) but may in certain instances serve that purpose. However, a specific example of such a calcining/drying is also set out below.

The silica gel obtained was calcined in an oven at l,000 F. for 4 hours before evaluation. The physical properties of the silica xerogel thus obtained were:

SA=268 m. /g., PV=2.52 cm. /g., Av. PD=365 A.

The dried silica gel contained less than 1,000 p.p.m. sodium, based on the total silica gel weight.

Referring to FIG. 1 of the drawings, it will be seen that the major portion of the cumulative surface area of the silica xerogel product is accounted for by gel having a pore size distribution in the narrow range of from 300 to 600 A. FIGS. 6 and 7 show the same results in terms of cumulative percent pore volume and cumulative pore volume, respectively. The expression cumulative pore volume indicates that pore volume resulting from the summation of volumes of all pores in one gram of gel having pore diameters within a stated range or at a certain value.

After calcination in a fluidized bed at 1,800 F. for 3 hours to activate the gel for use as a catalyst, the physical properties were unchanged within the limits of analytical error:

SA=278 m. /g., PV=2.54 cm. /g., Av. PD=365 A.

Such gels are particularly useful for olefin polymerization.

Other activation temperatures may be used.

EXAMPLE II This example illustrates the interrelationship between the shrinkage of the pore volume of the gel and the water content in the gel subsequent to solvent displacement with the substantially completely water-miscible organic solvent of the present invention but prior to the initiation of drying.

The silica gel prior to the solvent displacement step was formed as in Example I.

The product was re-slurried in acetone and washed with acetone until the water in the acetone titrated a certain percentage.

The acetone was then decanted off and the hydrogel dried. The percent pore volume shrinkage was measured at various water contents prior to drying. Those values were plotted in FIG. 5.

Two methods of investigation were used to derive the data for FIG. 5. One method assured equilibrium conditions of the solvent-water mixture in the pores of the gel with the body of the liquid. The second method hindered equilibrium conditions.

In both methods of investigation, the percent pore volume shrinkage was based on the ultimate pore volume which could be obtained with unlimited solvent washing. For instance, a percent pore volume shrinkage of 10% indicates that the pore volume was 10% smaller than the ultimate pore volume obtainable.

Referring now to FIG. 5 of the drawings, the weight percent of water which remained in the acetone after solvent displacement with acetone (the amount of water titrated) is plotted against the percent pore volume shrinkage. Line A represents the equilibrium values, whereas line B represents the hindered, or non-equilibrium, values.

Some of the values used to plot the curves are numerically presented below:

TABLE 1 [Relationship of percent water in solvent body to the pore volume of the resulting dried gel after 2% hours agitation (equilibriumfl {Relationship of percent water in solvent body to the pore volume of the resulting dried gel after about P hour agitation (nonequilibnumfl Pore Percent Percent water by volume, PV weight ec./g shrinkage The simplest way to displace water from the gel, and the procedure used in this example, is merely to immerse the gel in acetone after decanting all possible water therefrom. Water will be'displaced from the gel into the acetone until equilibrium is reached. At equilibrium the water content in the pores of the gel is equal to the water content in the body of the acetone-water, and in dynamic balance therewith.

To remove further amounts of water from the gel, if the equilibrium water percentage in the body of the acetone-water wash denotes an unacceptable percent pore volume shrinkage, it is necessary to upset the equilibrium conditions, and this is most easily done by decanting the first acetone-water wash and replacing this with pure acetone (or at least acetone of a lower water content). The pure acetone displaces additional water from the pores of the gel until a new equilibrium value is reached. The water content in the surrounding body of the acetonewater wash' is then determined and the percent pore volume shrinkage which will result can be found. If the shrinkage is too high, the above process is repeated with ever decreasing equilibrium water percentages until an acceptable percent pore volume shrinkage is reached, whereafter washing is discontinued.

The above process is essentially a repeating batch wash. It is preferably conducted under conditions of agitation to insure good contact between the acetone and the gel. A continuous wash could also be used where a stream of acetone is passed over the gel wherein the water content in the acetone after Washing is continuously monitored. The batch procedure is much simpler, however, since even in its preferred embodiment one needs only a tank and an agitator with simple discontinuous monitoring equipment to determine when equilibrium is reached.

The equilibrium point for any wash, be it batch or continuous, is easily determined; it is that point at which the water content in the body of the solvent surrounding the gel no longer changes with the passage of time. As a practical matter, this is determined to occur when two successive titrations provide a constant water percentage. If the water content in the surrounding solvent body 'increases from one measurement to the next, equilibrium conditions have not yet been reached. The spacing of time intervals is not important and depends, obviously, on the accuracy and sensitivity of the measuring equipment used. If the measuring equipment is sensitive enough a constant water content can be determined by very closely spaced measurements. For ordinary industrial operation using standard titration equipment, usually the measurements are about 30 minutes apart.

Nothing significant is gained by washing the gel beyond the time when the equilibrium state is reached, as once the equilibrium state is reached no additional water will be displaced (until, of course, equilibrium conditions are upset by, e.g., introducing fresh acetone to thereby permit the establishment of a new equilibrium level at a lower water content). Usually a washing for at most about 2 hours is sufficient to reach an equilibrium stage in batchtype processing.

By permitting equilibrium to be reached between the water in the pores of the gel and the Water in the surrounding body of displacing solvent it is possible to predict the percent pore volume shrinkage upon drying of the solvent from the gel. The percent pore volume shrinkage which is acceptable will vary depending upon the requirements of the gel product. For instance, assuming a 2:8 cm. g. p-re-drying pore volume, if one desired a 2. 8 cm. /g. dried product, one would have to wash to substantially zero water. On the other hand, if a final dried pore volume of 2.5 cm. /g. was acceptable, one could dry at an equilibrium water percent of about 8% and tolerate a 10 percent pore volume shrinkage. Since, however, most users will desire a gel with a dried pore volume of 2.50-2.90 cm. /g., one will generally wash to obtain a water content of less than 5%, e.g., in the range of to or 1% to 5% at equilibrium, to thereby obtain a lower than 6 percent pore volume shrinkage. Little is to be gained by permitting large amounts of water to remain prior to drying, and seldom will very high amounts of water, e.g., over about 47%, be permitted to remain in the gel after solvent displacement. The most preferred water content prior to drying is in the area of about 2% to about 3% by weight water in the displacing solvent, e.g., acetone.

Reference to FIG. 5 will enable one to easily determine the permissible Water content at equilibrium for any desired percent pore volume shrinkage. Although this plot is in terms of water in acetone, thesame data trend is applicable to other substantially completely water miscible displacing solvents, as the critical factor is the water content in the pores of the gel and this factor can be determined on a percent by weight basis no matter what displacing solvent is selected.

From the heretofore offered discussion, it will be clear that it is not mandatory to wash to an equilibrium value. The benefit provided by washing to an equilibrium value is that one can predict the percent pore volume shrinkage in advance of drying, a necessity for practical commercial operation where different runs may be necessary to produce silica gels of differing pore volumes for different consumers.

To summarize, one can predict the permissible amount of water in the gel at drying for any given set of gel formation conditions by the following procedure:

1. wash a first sample of the gel to zero equilibrium water and determine the ultimate dried pore volume;

2. wash further samples to decreasing sequential amounts of water from the highest water content of interest, and, at equilibrium between the water in the gel and the water in the surrounding body of displacing solvent, de-

'termine the water content in the surrounding body of solvent, dry the gel and measure the pore volume;

3. calculate the percent pore volume shrinkage, i.e., 100- [*(pore volume/ ultimate pore volume) X 100];

4. plot the percent pore volume shrinkage vs. equilibrium water content.

From the above discussion it is to be clearly understood that the present process is not limited to any maximum percent pore volume shrinkage. On the contrary, in some instances a very high percent pore volume shrink-age will be acceptable, and in others a very low percent pore volume shrinkage must be observed, depending upon end use requirements.

EXAMPLE II'I 20,160 g. of sodium silicate solution contain-ing 28.7% S10 and 8.9% Na O was added to 25,440 g. of H 0 and the mixture cooled to 5 C. under agitation.

21.305 g. of H (12.75 wt. percent) was then added as follows:

8,520 g. was added in one hour and the remaining amount in 45 minutes. The final pH of the slurry was 5.0.

The slurry was then heated to C. within 2 hours and maintained at 95 C. for 2 hours at a pH of 5.0.

-A 30 g. sample was drawn and washed with a solution of 10 g. of NH NO in 3,000 ml. of H 0, then with deionized Water until the wash liquor titrated or contained less than 20 p.p.m. Na SO at equilibrium. To illustrate all the features of the process, the product was homogenized. 10 g. of the resulting slurry containing 8.5% solid was vacuum freeze dried using a New Brunswick Model B-67 freeze dryer. The cold trap was set at C. and the sample, pre-frozen, maintained at -40 F.

The slurry can also be homogenized prior to washing with NH NO Homogenization prior to washing (if homogenization is necessary) is most preferred as smaller panticles are easier to wash to the required sodium level.

In a preferred embodiment, for certain end uses, the following optional step can be performed in the process of this invention:

Calcining the product from step 4 at a temperature between about 500 F. and about 1,200 E, preferably at about 1,'000 F. It should be noted that this final calcin'ation/drying at 500-l200 F. is generally not considered a catalyst activation (which normally requires higher temperatures) but may in certain instances serve that purpose. A specific example of such a calcining/drying is also set out below.

1 mg. of sample was recovered and heated or calcined in an oven at 1000 F. for 4 hours before evaluating it. Physical properties of the silica gel obtained were:

SA=3 21 m. /g., PV=2.49 cm. /g., PD=312 A.

Calcination in a fluidized bed for 3 hours at 1800 F. to activate the gel for use as a catalyst did not alter the physical properties within the limits of analytical error:

SA=329 m. /g., PV=2.47 cm. /g., PD=301 A.

Other activation temperatures may be used.

Referring to FIG. 7 of the drawings, which is a distribut-ion curve of the pore sizes, it will be seen that the major portion of the cumulative pore volume of the silica gel product is accounted for by gel having a pore size distribution in the narrow range of from 300 A. to 600 A. FIGS. 1 and 6 show the same results in'terms of cumulative percent of surface area and cumulative percent pore volume, respectively.

EXAMPLE IV To compare the freeze drying technique to the water displacement technique, a 30 g. sample was prepared as in Example III and was washed until the filtrate titrated less than 20 p.p.m. Na SO The product was then homogenized, reslurried in acetone and washed with acetone by decantation until the water in the acetone titrated less 1 1 than 1% at equilibrium between the wash liquid and the liquid in the pores of the gel.

The acetone was distilled off and the resulting silica gel was calcined in an oven at 1000 F. for 4 hours.

Physical properties of the silica gel obtained were found to be substantially the same as those obtained by the freeze drying technique in Example III, i.e., the properties were SA=327 m. /g., PV=2.61 cm. /g., PD=3l9 EXAMPLE V This Example illustrates the azeotropic distillation water removal technique, which is generally performed as follows.

The azeotropic distillation is conducted by admixing a solvent with the wet gel in an amount in excess of the azeotropic mixture. The admixture is then alternatively refluxed for 0.5 to 2 hours and distilled for a total cycle time of between about 5 to 20 hours and preferably about 8 hours. The temperature of the distillation is dependent upon the particular solvent used and the pressure of the system.

A preferred method of carrying out this distillation process is shown in FIG. 8. In this process, the hydrogel is charged into the jacketed vessel 1 through valve A. A solvent which is non-miscible with water and which forms an azeotrope with water, is retained in storage tank 3 and is pumped into vessel 1. The preferred quantity of solvent is an amount equal to the amount of water contained in vessel 1, although smaller quantities can be .used. Vessel 1 is then heated to provide the required distillation.

The distillate is recovered through heat exchanger 2 into storage tank 3 where the azeotrope separates into two phases and water is removed by decantation.

The solvent portion is recirculated into the system by being continuously pumped back into vessel 1 and the system is operated for between about 2 to 20 hours, preferably for about 5 hours, after which pump 4 is stopped and excess solvent is distilled until the silica gel is dried.

Two high and low capacitance probes 5 and 6 are located at the bottom of tank 3 which activate motor valve 7 draining the water from the tank through line 8.

The dry silica gel is discharged through valve B.

Adequate solvents for the azeotropic distillation method include: benzene, toluene, xylene, ethyl-acetate, n-propyl formate, n and iso-butyl formate, n and iso-amyl formate, benzyl formate, n-propyl acetate, n and iso-butyl acetate, 11 and iso-amyl acetate, benzyl acetate, phenyl acetate, ethyl propionate, n-propyl propionate, iso-butyl propionate, iso-amyl propionate, methyl butyrate, n-propyl butyrate, n and iso-butyl butyrate, isoamyl butyrate, methyl iso-butyrate, ethyl iso-butyrate, di-iso-butyl ether, di-isoamyl ether, diphenyl ether, phenetole, anisole, resorcinol diethyl ether and other solvents which are non-miscible with water and which form azeotropic mixtures when distilled with water.

A specific example of a process employing the azeotropic distillation technique is as follows:

20,160 g. of sodium silicate solution containing 28.7% SiO and 8.9% Na O was added to 25,440 g. of H and the mixture cooled to C. under agitation.

21,305 g. of H SO (12.75 wt. percent) was then added as follows:

8,520 g. was added in one hour and the remaining amount in 45 minutes. The final pH of the precipitate was 5.0.

The slurry was then heated to 95 C. within 2 hours and maintained at 95 C. for 2 hours, washed with a solution of 2,225 g. of NH NO in 45 gallons of water, then with deionized water until the wash liquor titrated or contained less than p.p.m. Na SO at equilibrium. The slurry was at pH 5.0.

To illustrate all features of the invention, the product slurry wasthen homogenized for 30 minutes after which a 300 g. sample was drawn, allowed to settle and the excess water decanted. The slurry can also be homogenized prior to washing with NH NO or can be homogenized after being azeotropically distilled. Homogenization prior to washing (if homogenization is necessary) is most preferred as smaller particles are easier to wash to the required sodium level. Homogenization after azeotropic distillation is non-preferred due to the extra care required with flammable vapors.

60 g. of slurry, containing 11% by weight of solids, was recovered and added to 1,500 g. of ethyl acetate in a distillation flask. The azeotrope was then distilled at atmospheric pressure by using the following procedure: alternatively distilling for /2 hour and refluxing for 1% hours for a total process time of 8 hours. For precaution, the solvent was continuously added to maintain the same level of liquid in the flask. Temperature fluctuations during the azeotropic distillation were between 70.5 C. and 71 C. After 8 hours, the gel was dried by distilling the remaining solvent.

In a preferred embodiment, for certain end uses, the following optional step can be performed in the process of this invention:

6. Calcining the product from step 5 at a temperature between about 500 F. and about 1,200 E, preferably at about 1,000" F. It should be noted that this final calcination/drying at 500-1,200 F. is generally not considered a catalyst activation (which normally requires higher temperatures) but may in certain instances serve that purpose. However, a specific example of such a calcining/ drying is also set out below.

The silica xerogel recovered was calcined in an oven at 1,000 F. for 4 hours before evaluating.

Analysis of the physical properties were:

SA=307 m. /g., PV=2.35 cm. /g., PD=317 A.

The pore size distribution of this product is shown in FIGS. 1, 6 and 7.

EXAMPLE VI A 1,000 g. sample obtained as in Example V after homogenization was drawn, allowed to settle and the excess water decanted.

A 200 g. sample of the slurry containing 11% by weight solids was then added to a 2,000 ml. distillation fiask installed in a laboratory set-up equivalent to the one represented in FIG. 8. 1,000 g. of ethyl acetate was pumped into the distillation flask before starting the distillation, and the distillation was run for 5 hours. The ethyl acetate continuously recovered was pumped back into the distillation flask after the water had been separated by decanation, at a rate equal to the distillation rate of the solvent phase.

After 5 hours, the pump was stopped and the excess solvent distilled off as to obtain a dried silica gel.

The xerogel recovered was calcined in an oven at 1,000 F. for 4 hours and evaluated for its physical properties.

SA=323 m. /g., PV=2.53 cm. /g., PD=313 A.

FIG. 7 is a distribution curve of the pore sizes. It can be seen that the major portion of the pore volume of the silica xerogel product is accounted for by the gel having a pore size distribution in the narrow range of from 300 A. to 600 A.

Calcination in a fluidized bed for 3 hours at 1,800 F. to activate the xerogel for use as a catalyst base did not alter the physical properties, within analytical error, which were:

SA=329 m. /g., PV=2.47 cmF/g, PD'=301 A.

Other activation temperatures can be used.

13 EXAMPLE VII SA=342 m. /g., PV=1.91 cm. /g., PD=223 A.

As can be seen, when using an azeotrope distillation without using the conditions described in Example V or Example VI to dry the product, the pore volume and consequently the average pore diameter were adversely affected.

EXAMPLE VIII To compare the azeotrope distillation technique to the water displacement technique, a 30 g. sample was prepared as in Example V and was washed until the filtrate titrated less than 20 p.p.m. N SO The product was then homogenized, reslurried in acetone and washed with acetone by decantation until the water in the acetone titrated less than 1% at equilibrium.

The acetone was distilled off and the resulting silica gel was calcined in an oven at 1,000 F. for 4 hours.

The physical properties of the silica obtained were found to be substantially the same as those obtained by the azeotropic distillation technique (ExampleV):

SA=333 m. /g., PV=2.50 cm. /-g., Av. PD=300 A.

EXAMPLE IX In this example, a silica gel produced as in Example III was tray dried at 80 (3., rather than employing the freeze drying technique described therein.

Physical properties of the gel were:

SA=110 m. /g., PV=0.41 cm. /g., and PD=149A.

EXAMPLE X All of the variables during precipitation, except final SiO concentration, were maintained as in Examples I, III and V. Final SiO concentrations of 7% and were chosen and FIG. 2 shows the effect of SiO concentration on the porosity of the gel just after precipitation.

, Referring to FIG. 2 of the drawings, it will be seen that relatively lower pore volumes are obtained where the final silica concentration in the slurry is 7% and 10% as compared with the pore volume obtained in Example I where an Si0 concentration of about 8.5% was obtained. FIG. 2 also demonstrates that the surface area increases as the SiO concentration increases. Thus, in order to get maximum pore volume and at the same time obtain a good surface area level it is preferable to maintain the SiO concentration at between about 8 and 9% and most preferably at about 8.5%.

The preferred sodium silicates used can be represented by the formula xNa O.ySi0 where x/y is from 1/1 to about 4/1. Corresponding ranges for other water soluble silicates will be easily calculatable to one skilled in the art.

EXAMPLE XI All variables during precipitation, except the acid addition rate, were maintained as in Examples I, III and V. The initial 40% of the stoichiometric acid requirement was added at a constant rate in 0.5 and 2.0 hours as compared to 1.0 hour in Example I.

Example XI clearly illustrates, as is shown in FIG. 3

of the drawings, that both pore volume and surface area are dependent upon the rate at which the neutralizing medium, in this case the sulfuric acid, is added to the aqueous silicate solution. It will be seen that optimum pore volume and surface area values are obtained using an addition rate of about 1 hour for 40% of the stoichiometric amount, the get point for this particular system, and that lower values for pore volumes and surface areas are obtained where more rapid or slower addition rates are employed.

EXAMPLE XII All variables during precipitation, except temperature, were maintained as in Examples I, III and V. Temperatures of 25 C. and 50 C. were used during precipitation. FIG. 4 shows the effect of temperature on the porosity of the gel just after precipitation.

' As will be seen by reference to FIG. 4, the pore volume decreases as the precipitation temperature increases. Surface area also tends to decrease with an increase in the precipitation temperature, but this value flattens out at about 5 C. and this is the preferred temperature of precipitation in order to secure optimum values for pore volume and surface area.

EXAMPLE XIII In this example all the variables during the precipitation were maintained as in Examples I, III and V, but CO was used to neutralize the sodium silicate instead of sulfuric acid. The physical properties of the silica gel were similar to those obtained in Examples 1, III and V.

EXAMPLE XIV All the variables during precipitation were maintained the same as in Examples I, III and V. After precipitation, the gel was adjusted to a pH of 9.0 by adding NaOH (2-normal) and was heated for one hour at 95 C. The

gel was then washed and the water removed as in Examples I, III and V before evaluating it.

The properties of the silica gel were as follows:

SA=346 m. /g., PV=1.96 cm. /g., and Av. PD=226 A.

As will be seen by comparing the properties of the gel obtained in accordance with Example XIV with the products of Examples I, III and V too high a pH and relatively brief heating during the ageing step results in a loss of pore volume.

EXAMPLE XV TABLE 3 SA PV Avg.

(ml/g.) (cmfi/g.) PD (A.)

As will be seen from the data reported in Table 3, the thermal ageing of the precipitate should be conducted for a minimum of about 1 hour when the pH of the slurry is 5.0 in order to obtain a silica xerogel product having an average pore diameter in the desired range of from about 300-600 A.

1 EXAMPLE XVI In this example, all of the variables during precipitation and heating were maintained the same as in Examples I, III and V. The gel was then washed as in Examples I, III and V, but only until the filtrate titrated 100 p.p.m. Na SO The water was then removed as in Examples I, III and V, and the product evaluated. The physical properties of the gel were substantially the same as were found for the products of Examples I, III and V. However, when the product was calcined in a fluidized bed at 1,825 F. for 3 hours and evaluated, the properties were as follows:

SA=225 m. /g., PV=1.62 cm. /g., Av. PD=286 A.

As will be seen from the porosity of the product obtained in accordance with Example XVI when compared to the products of Examples I, III and V, after calcination in a fluidized bed at l,800 F., the surface area, pore volume and average pore diameter are all adversely affected by the relatively high level of Na SQ, left in the product.

What is claimed is:

1. A method for producing a silica xerogel having a narrow pore diameter distribution primarily within the range of from 300-600 A., a surface area within the range of from 200-500 m. /g., a pore volume of from about 2.0 to about 3.5 cm. g. and stability at temperatures up to about 2000 F. comprising:

(a) precipitating a silica hydrogel, under conditions of good agitation, by the neutralization of an aqueous alkaline silicate solution to provide a silica hydrogel slurry having an SiO concentartion of from 5% to 12% by weight, the temperature during precipitation being maintained at from about 0 C. to about 17 C., and the neutralizing medium being added to the aqueous alkaline silicate solution at a rate such that the gel point of the solution is reached in from about 30 to about 120 minutes, whereafter the remaining amount of the neutralizing medium required to provide an end pH of between pH 3 and pH 8 is added,

(b) maintaining the hydrogel slurry from step (a) at a pH within the range of from 3 to 8 at a temperature and for a time sufficient to strengthen the hydrogel structure,

(c) reducing the alkaline material concentration in the product of step (b) by washing the product of step (b) with a liquid which displaces said alkaline material so that the wash liquor, in equilibrium with the product of step (b), contains less than about 20 ppm. of the alkaline material expressed as an acidbase salt thereof, and

(d) removing the water from the product of step (c) in a manner which exerts a surface tension on the gel, during removal, which is less than the surface tension exerted upon the gel by the direct conversion of liquid water to water vapor.

2. The method of claim 1 wherein the product of step (d) is calcined.

3. The method of claim 1 wherein the precipitation of the silica hydrogel in step (a) is accomplished by neutralization of the aqueous solution with an agent selected from the group consisting of a strong acid, a weak acid and an ion-exchange resin.

4. The method of claim 1 wherein the alkaline silicate is sodium or potassium silicate and the temperature during step (b) is maintained above the temperature during step (a).

5. The method of claim 1 wherein the SiO concentration in the hydrogel slurry of step (a) is from about 8% to about 9% by weight.

6. The method of claim 1 wherein the temperature during neutralization step (a) is from about 0 C. to about 10 C.

7. The method of claim 1 wherein the end point of step (a) is reached when the pH of the hydrogel slurry is from about 5 to about 7.

8. The method of claim 1 wherein the pH of the hydrogel slurry in step (b) is from about 4 to about 7.

9. The method of claim 1 wherein the temperature of the hydrogel slurry during step (b) is maintained at about 50 C. to about C.

10. The method of claim 1 wherein the time of step (b) is about 1 to about 6 hours.

11. The method of claim 1 wherein the reduction of concentration of alkaline ions is accomplished by washing with a soltuion of displacing salts.

12. The method of claim 11 wherein said salts are selected from the group consisting of ammonium chloride, ammonium nitrate or mixtures thereof.

13. The method of claim 1 wherein step (c) is accomplished by washing with a solution of mineral acid selected from the group consisting of HCl, H 80 and HNO 14. The method of claim 1 wherein up to about 40% of the neutalizing medium is added in from about 30 to minutes and the remaining neutralizing medium is added in about 20-90 minutes more.

15. The method of claim 1 wherein step (b) is conducted under agitation.

16. The method of claim 1 wherein the silicate is a sodium silicate of the formula xNa O-ySiO in which y/x is from 1/ 1 to about 4/1.

17. The method of claim 1 further comprising comminuting the hydrogel after step (b) but prior to step ((1).

References Cited UNITED STATES PATENTS 2,700,061 1/1955 Owen 252-62 X 2,731,326 1/1956 Alexander et al. 423-338 2,765,242 10/ 1956 Alexander et al. 252-313 X 2,785,051 3/1957 Miller 423338 3,041,140 6/1962 Alexander 423335 3,081,154 3/1963 Acker et al. 423339 3,367,742 2/1968 Marotta et al. 423-335 3,428,425 2/1969 Marotta 423339 3,453,077 7/1969 Hyde 423338 EDWARD STERN, Primary Examiner US. Cl. X.R. 

